Many people have misunderstood the miracle of Yam Souf. Helped by Hollywood movies, the image of the miracle has only been solidified in our minds: Moshe Rabeynou splits the waters of Yam Souf in half. Bnei Yisrael started on one side of the sea, and ended up on the other. Unfortunately, this never occurred.
It is important to always return to our Jewish sources, rabbis who were and still are the pillars of our faith- Baaleh HaTosfot, Rambam, The Tour etc. In the Gemara in Erachin, the Tosfot describe that the water was split in a semi circle fashion, cutting into the center of the ocean but then returning to the same side. Twelve tunnels of water were made, each starting on the same side of the sea, and then returning back to the same side, further down the shoreline.
[It may be difficult to picture in your mind. Imagine this letter U upside down. Then imagine another upside down U, slightly smaller, placed inside the first letter. This is the first of the twelve paths or tunnels that Hashem created for the tribes of Israel to walk through. Continue to create smaller upside down U's until twelve paths are made. This was the way the miracle appeared. (For an image you can open Gemara Avodah Zara to the back of the book, the perush of the Rambam on Avot, Fifth Perek.)]
The purpose of creating this miracle was to destroy the Egyptians chasing after them, and not to get Bnei Yisrael any place new. A careful reader of the text will note that Bnei Yisrael are in the same wilderness that they started in in the following parashiyot.
We can understand now that the miracle was actually greater than one has ever thought. Because the inner tunnel of water was shorter than the largest, we must say that the Egyptians who followed the tribe of Yisrael into that tunnel started to drown while other tribes were still walking through their larger paths. This part of the miracle is hinted to in the morning prayer in Ezrat Avotenou, we say that Hashem split the sea, drowned our enemies and Bnei Yisrael walked the through. The order is now understood, how Egyptians were drowning while their were still those of Bnei Yisrael walking through the Yam Souf.
-Sources and ideas compiled in Hagadat Hegyoneh Haggadah written by Rabbi Y. Mirsky of the Sephardic Rabbincal College.com